avcodec/idctdsp: move add/put_pixels_clamped_c to header

This allows sharing them with the xvid IDCT

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavc: stop exporting internal functions and tables

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>